Gastrointestinal bleeding is a somewhat common and serious condition that can be fatal if left untreated. This problem has prompted the development of a number of endoscopic therapeutic approaches to achieve hemostasis, such as the injection of sclerosing agents and contact thermo-coagulation techniques. Although such approaches can be effective, bleeding often continues for many patients and corrective surgery therefore becomes necessary. Because surgery is an invasive technique that can be associated with many undesirable side effects, there exists the need for highly effective, less invasive procedures.
Mechanical hemostatic devices have been used in various parts of the body, including gastrointestinal applications. Such devices are typically in the form of clamps, clips, staples, sutures, etc. which are able to apply sufficient constrictive forces to blood vessels so as to limit or interrupt blood flow. One of the problems associated with conventional hemostatic devices, however, is that they can only be delivered using rigid-shafted instruments via incision or trocar cannula. Moreover, none of the conventional endoscopic hemostatic devices are strong enough to cause permanent hemostasis.
In order to avoid the problems associated with conventional hemostatic devices, the use of shape memory alloys has been proposed. For example, U.S. Pat. No. 4,485,816, hereby incorporated by reference, discloses the use of a shape memory surgical staple for use in holding the edges of a wound together while it heals. Similarly, U.S. Pat. No. 5,002,563, hereby incorporated by reference, discloses the use of shape memory sutures.
Shape memory alloys (SMA's) have the ability to "remember " specific shapes which corresponds to particular metallurgical phases. If deformed, SMA's can be heated or cooled to invoke a phase transformation, which in turn, causes a transformation in shape. Shape memory alloys are characterized by a transition temperature or transition temperature range, above which the predominant metallurgical phase is termed austenite and below which the predominant phase is termed martensite. The transformation temperatures of SMA's are commonly discussed with reference to M.sub.s and M.sub.f, the martensitic start and finish temperatures, respectively, and A.sub.s and A.sub.f, the austenitic start and finish temperatures, respectively. The transformation between these phases is reversible such that when alloys are deformed into some first configuration while in the austenitic state, cooled into a martensitic state, deformed into a second configuration, and then re-heated to the austenitic state, the alloy will revert back to the first configuration by virtue of the martensite-to-austenite phase transformation.
PCT Publication No. WO 96/16603, hereby incorporated by reference, specifically discloses the use of shape memory materials to address the problem of gastrointestinal bleeding. In this reference, a hemostatic staple is employed to affect hemostasis of an actively bleeding peptic ulcer. The staple makes use of the thermally-induced martensite-to-austenite transformation in shape memory nickel-titanium alloys ("nitinol"), thus requiring the application or removal of heat to the staple for deployment. One of the problems with this and similar SMA devices is that the change in temperature necessary to induce the required shape change can be procedurally difficult, and more importantly, can put the nearby tissue and surgical instrumentation at risk. In addition, it can be difficult to manufacture SMA's with the precise transformation temperatures necessary for surgical applications. It is therefore necessary to carefully monitor the temperature of such devices during shipping and storage so as to avoid phase transformations during this time. Moreover, the thermally-induced phase change may not produce forces adequate to hemostatically close vessels or compress tissue.
The use of nitinol alloys having the ability to form stress-induced martensite as opposed to thermally-induced martensite has been used in medical devices so as to avoid the potential problems of SMA devices. In such alloys, the reversible transformation between martensite and austenite occurs by the application and removal of stress rather than heat. Such alloys are characterized by an M.sub.d temperature, which is greater than A.sub.s and represents the maximum temperature at which stress-induced martensite can form. By deforming these alloys at a temperature between A.sub.s and M.sub.d, the alloy transforms from its austenitic phase to a stress-induced martensitic phase. Upon release of the stress within this temperature range, the alloy reverts back to its austenitic phase and unstressed configuration. The property of nitinol which allows it to be deformed in its austenitic state so to cause a transformation to stress-induced martensite that is transformed back to austenite by the release of stress is often termed "pseudoelasticity. " Strains of 8% or more are obtained in pseudoelastic nitinol, thus making this material useful for a wide range of applications where a large amount of recoverable deformation is required.
U.S. Pat. No. 4,665,906, incorporated herein by reference, describes some medical devices which make use of pseudoelastic nitinol. In such devices, austenitic nitinol is deformed to form stress-induced martensite and held in its deformed configuration and martensitic state by a restraining member. In this condition, the device is introduced into the body, where it is removed from the restraining member to return to its austenitic state and configuration.